[unreadable] Autosomal recessive diseases are by definition those where only individuals with two mutated copies of the disease genes are affected. However, even in these diseases, there is often some manifestation in the heterozygous carriers. While there are usually no marked phenotypes in carriers, they often have subtle phenotypes that are minor differences from non-carriers. Most autosomal recessive diseases are rare but carriers are not. All individuals are carriers of several deleterious mutations. These mutations are likely to contribute significantly to the wide variation in phenotype among us, from disease susceptibility to variation in response to stress. [unreadable] [unreadable] In this project, we will study the carriers of radiosensitivity syndromes in order to understand the individual variation in response to radiation. We will focus on the gene expression profiles of heterozygous carriers of four radiosensitivity syndromes: Ataxia Telangiectasia, Nijmegen Breakage Syndrome, Bloom Syndrome and Fanconi Anemia. Physical examination and standard biochemical tests do not reliably detect the subtle phenotypes in these carriers. Our previous work (Watts et al, 2002) establishes that heterozygous carriers of Ataxia Telangiectasia have a "gene expression phenotype." In this project, we will extend and determine whether carriers of other radiosensitivity syndromes also have expression phenotypes at baseline and in response to ionizing radiation. The specific aims are: 1) Identify the expression phenotype of carriers of Ataxia Telangiectasia, Bloom Syndrome, Nijmegen Breakage Syndrome and Fanconi Anemia at baseline; 2) Characterize the expression phenotype of carriers of Ataxia Telangiectasia, Bloom Syndrome, Nijmegen Breakage Syndrome and Fanconi Anemia in response to ionizing radiation (IR). [unreadable] [unreadable] The results from this study will have important implications for understanding the basis of variation in radiation response. The approach can also be broadened to study the contribution of heterozygosity of recessive diseases to the complex genetic architecture of human diseases and traits. [unreadable] [unreadable]